The present invention relates to the preparation of concentrated aqueous suspensions of calcium carbonate by crystallizing calcium carbonate in an aqueous medium.
In order to reduce the storage bulk of calcium carbonate crystals intended especially for the paper, plastics and paint industries, and, furthermore, in order to facilitate their handling, it is advantageous to store and transport them in the form of concentrated aqueous suspensions generally containing from 65 to 80% by weight of crystals.
For this purpose, Belgian Pat. No. 819,747 of Gewerkschaft Victor Chemische Werke, filed on the 10th of September 1974, proposes a process in which calcium carbonate is crystallized in an aqueous medium, the resulting aqueous suspension of calcium carbonate is filtered in order to separate a wet concentrate of calcium carbonate therefrom, the calcium carbonate concentrate is fluidized and homogenized in the presence of a dispersing agent and the concentrated suspension of calcium carbonate thus obtained is ground wet.
This known process exhibits the disadvantage that it requires an expensive grinding operation. In fact, this operation is necessitated by the need to break up the numerous coarse agglomerates of calcium carbonate crystals present in the wet concentrate of calcium carbonate. In addition to its unfavorable influence on the cost of the process, this grinding operation exhibits the disadvantage that it gives rise to the formation of calcium carbonate fines which are not generally suitable as a filler.
An attempt has been made to omit the grinding operation by crystallizing the calcium carbonate in an aqueous medium containing an alkali metal salt or alkaline earth metal salt of nitrilotriacetic acid, as a dispersing agent, and by then filtering the resulting dilute suspension of calcium carbonate, as described in Published German Patent Application No. 1,116,203, filed on the 13th of October 1959 in the name of Farbenfabriken Bayer AG.
The filter cakes obtained by this known process are in the form of aqueous suspensions of fine crystals which are very suitable as a filler in papermaking, plastics and paint. However, the concentration of calcium carbonate crystals in these aqueous suspensions is generally insufficient to enable them to be stored and handled economically, with the result that it is necessary to concentrate them by evaporation.